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A304279 Numbers equal to the sum of their aliquot parts, each of them increased by 8. 8
188, 370, 568, 1612, 1648, 1892, 4832, 70384, 165632, 430508, 2394848, 76393148, 85532348, 174712264, 540655616, 2359808828, 2544998588, 2199366139904, 35236711282688, 2931045725111036, 20542625035648508, 144115310213988352, 144141640382529536 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Searched up to n = 10^12.

a(24) > 10^18. - Hiroaki Yamanouchi, Aug 28 2018

From Giovanni Resta, May 11 2018: (Start)

If p = 2^(1+t) + (1+2*t)*k - 1 is a prime, for some t > 0 and k even, then x = 2^t*p is in the sequence where k is the value by which the sum of aliquot parts is increased.

In this sequence, k = 8; for t = 20 we get 2199366139904, which is a term greater than 2544998588, but this does not exclude the existence of other intermediate terms following a different solution pattern.

In fact, there could also be sporadic solutions of the type x = 2^t*r*q, where r and q are prime and for which no closed form is known. E.g., for k = 8 we have x = 2^14*32771*268460149.

To find them, since d(n) = 4*(t+1) and sigma(n) = (2^(t+1)-1)*(1+r)*(1+q), the relation 2*n = sigma(n) + k*(d(n)-1) becomes 2^(t+1)*r*q = (2^(t+1)-1)*(1+r)*(1+q) + k*(4*t+3), which, for fixed t and k, is a quadratic Diophantine equation in r and q that could admit solutions with r and q prime.

(End)

Terms using odd values of k seem very hard to find. Up to n = 10^12, only three such terms are known: 2, 98, and 8450, for k = 1, 5, and -7, respectively.

LINKS

Table of n, a(n) for n=1..23.

EXAMPLE

Aliquot parts of 188 are 1, 2, 4, 47, 94; (1+8) + (2+8) + (4+8) + (47+8) + (94+8) = 188.

Aliquot parts of 370 are 1, 2, 5, 10, 37, 74, 185; (1+8) + (2+8) + (5+8) + (10+8) + (37+8) + (74+8) + (185+8) = 370.

MAPLE

with(numtheory): P:=proc(q, k) local n;

for n from 1 to q do if 2*n=sigma(n)+k*(tau(n)-1) then print(n);

fi; od; end: P(10^12, 8);

CROSSREFS

Cf. A000005, A000203, A000396, A304276, A304277, A304278, A304280, A304281, A304282, A304283, A304284.

Sequence in context: A243077 A186397 A260836 * A099945 A211814 A233786

Adjacent sequences:  A304276 A304277 A304278 * A304280 A304281 A304282

KEYWORD

nonn,hard,more

AUTHOR

Paolo P. Lava, Giovanni Resta, May 11 2018

EXTENSIONS

a(18)-a(23) from Hiroaki Yamanouchi, Aug 28 2018

STATUS

approved

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Last modified January 18 13:30 EST 2020. Contains 331007 sequences. (Running on oeis4.)